Processes for hydrogenating heavy hydrocarbon oil (hereinafter referred to simply as heavy oil) containing large amounts of asphaltene and/or heavy metals such as nickel and vanadium to convert the heavy oil into desulfurized hydrocarbon oil having a low asphaltene and/or heavy metals content (and great added value) have been widely investigated.
When heavy oil is hydrogenated, the loading of materials per unit quantity of catalyst which can poison the catalyst is high due to the presence of asphaltene and/or heavy metal compounds which are present at high concentrations in the heavy oil so that the activity of the catalyst is rapidly decreased in a short period of time. Asphaltene dispersed as colloidal particles in the heavy oil is a macromolecule. Thus, when conventional desulfurization catalysts are used, asphaltene has high resistance to the diffusion of oil into the catalyst particles and coke is formed in high quantities on the surfaces of the catalyst particles. Thus, catalyst activity is rapidly decreased and the course of the reactions is obstructed in a short period of time. Further, heavy metals such as nickel and vanadium in the heavy oil are deposited on the surfaces of the catalyst particles and the catalysts are also thereby poisoned, i.e., catalyst activity is rapidly decreased. Therefore, the catalysts must be replaced by fresh catalysts and costs are increased. In some cases, heavy oil is no longer treated at many plants.